1. Field of the Invention
The present invention relates to the field of television signal processing. More specifically, the present invention relates to the recovery of the leading run-in clock of a vertical blanking interval (VBI) scan line.
2. Background Information
Broadcasting of closed captions, teletext, etc. in the vertical blank intervals of television signals is known in the art. VBI scan lines are typically broadcast during the vertical blank intervals in accordance with one of a number of industry standard formats. For example, teletext scan lines are often broadcast in accordance with North American Broadcast Television Specification (NABTS).sup.1. Since hardware systems processing VBI scan lines typically operate with their own internal system clocks, virtually all industry standard formats call for the inclusion of a leading run-in clock at the beginning of a VBI scan line, to facilitate synchronization and decoding of the VBI scan line. For example, NABTS calls for a modulated data bit rate of 5.7272 Mhz, and the inclusion of 8 run-in clock cycles at the beginning of a teletext scan line, followed by phase-aligned modulated data bits to be decoded. FNT .sup.1 See Joint EIA/CVCC Recommended Practice for Teletext North American Basic Teletext Specification (NABTS), EIA-516, May 1988.
Conventional capture hardware would begin sampling the VBI scan line some lead time before the beginning of the run-in clock, using an oversampling ratio typically in the range of 3.0 to 6.0 times the data rate frequency, and continue capturing until the end of the VBI scan line. A conventional decoder would begin processing the VBI scan line at the beginning of the captured data, that is, some significant number of samples before the beginning of the run-in clock. From the oversampled run-in clock data, a conventional decoder would determine the average phase error of each clock peak, relative to a reference point, and adjust the reading of the data to be decoded accordingly, so that the readings are evaluated at the actual centers of the modulated data bit. If the average phase error, or any individual phase error, exceeds certain predetermined ranges, the VBI scan line will be rejected.
This conventional approach of starting processing a significant number of samples before the run-in clock, and using the average phase error of all run-in clock cycles as a basis for determining the proper phase adjustment, has a number of disadvantages:
a) substantial processing cycles can be wasted in processing the "pedestal" at the beginning of a VBI scan line before the first rising edge; PA1 b) the average phase error is distorted if the pedestal area contains noise which could be improperly interpreted as part of the run-in clock; PA1 c) the average phase error is exaggerated if one or more cycles of the run-in clocks were distorted by the presence of noise; and PA1 d) as a result, many otherwise good teletext scan lines wound up being unnecessarily rejected, simply because of the presence of a "large noise" in one or two of the run-in clock cycles, or before the beginning of the run-in clock cycles."
Thus, it is desirable if a more efficient as well as more tolerant or more precise run-in clock recovery approach can be found. As will be described in more details below, the present invention provides the improved approach, and achieves the above described desire as well as other desirable results, which will be readily apparent to those skilled in the art from the descriptions to follow.